1. Field of the Invention
The present invention relates to a golf ball having a cover formed from a thermoplastic polyurethane material; and more particularly to a golf ball having a cover formed from a thermoplastic polyurethane material which can be recycled for molding, which exhibits high restitution, and which exhibits excellent scuff resistance.
2. Description of the Related Art
In recent years, polyurethane materials have become of interest as materials for forming a golf ball cover. Polyurethane materials are classified into thermosetting polyurethane materials and thermoplastic polyurethane materials, and a process for forming a thermosetting polyurethane material into a product differs from a process for forming a thermoplastic polyurethane material into a product. A thermosetting polyurethane material can be formed into a product through the following procedure: a urethane prepolymer having an isocyanate end group and a curing agent such as polyol or polyamine, which serve as liquid raw materials, are mixed under heating; and the resultant mixture is fed directly to a mold and then heated, to thereby allow urethane curing reaction to proceed.
Many studies have heretofore focused on golf balls formed from thermosetting polyurethane materials. For example, U.S. Pat. Nos. 5,334,673, 6,117,024, and 6,190,268 disclose such golf balls. Meanwhile, U.S. Pat. Nos. 5,006,297, 5,733,428, 5,888,437, 5,897,884, and 5,947,843 disclose forming methods of thermosetting polyurethane materials.
Since a thermosetting polyurethane material exhibits no thermoplasticity, the material and a product formed from the material cannot be recycled. In addition, when a thermosetting polyurethane material is employed for forming a specific product such as a golf ball cover (i.e., a product which covers a core), efficient production of the product is not attained, since the heating curing step and the cooling step of the material requires long time, and high reactivity and instability of the material make control of the molding time very difficult.
In the case where a thermoplastic polyurethane material is formed into a molded product, the product is not directly obtained through reaction of raw materials, but is formed from a linear polyurethane material—an intermediate—which has been synthesized by employment of raw materials and a synthesis method, the raw materials and the method differing from those employed in the case of the aforementioned thermosetting polyurethane material. Such a linear polyurethane material exhibits thermoplasticity, and is cured through cooling. Therefore, such a polyurethane material can be molded by use of an section molding machine. Injection molding of a thermoplastic polyurethane material is a technique suitable for forming a golf ball cover, since the molding time of a thermoplastic polyurethane material is much shorter than that of a thermosetting polyurethane material, and a thermoplastic polyurethane material is suitable for precise molding. Meanwhile, a thermoplastic polyurethane material can be recycled, and is thus environmentally friendly. U.S. Pat. Nos. 3,395,109, 4,248,432, and 4,442,282 disclose golf ball formed from thermoplastic polyurethane materials.
However, when a golf ball cover is formed from a conventional thermoplastic polyurethane material, the resultant golf ball is not satisfactory in terms of feeling on impact, controllability, restitution, and scuff resistance upon being hit with an iron.
In order to solve much a problem, Japanese Patent Application Laid-Open (kokai No. 9-271538 discloses a golf ball cover formed from a thermoplastic polyurethane material exhibiting high restitution. However, the disclosed golf ball cover is not satisfactory in terms of scuff resistance upon being hit with an iron.
Japanese Patent Application Laid-Open (kokai) No. 11-178949 discloses a golf ball cover exhibiting relatively excellent scuff resistance upon being hit with an iron, which predominantly contains a reaction product formed from a thermoplastic polyurethane material and an isocyanate compound. When the cover is formed, an isocyanate compound such as a diisocyanate or a block isocyanate dimer, serving as an additive, is added to a thermoplastic polyurethane material in the course of heating, melting, and mixing by use of an extruder, or in the course of injection molding, to thereby allow reaction to proceed.
However, in the case of molding of the cover disclosed in Japanese Patent Application Laid-Open (kokai) No. 11-178949, since an isocyanate compound must be handled with great care due to its inactivation by moisture, obtaining a stable reaction product is difficult. Meanwhile, a block isocyanate exhibiting moisture resistance is not suitable for forming the cover, since a blocking agent issues a strong odor when the isocyanate is thermally dissociated. When an isocyanate compound assumes the form of powder or solution, control of the amount of the compound which is added to a thermoplastic polyurethane material is difficult, and therefore cover properties cannot be controlled adequately. In addition, since the thermoplastic polyurethane material differs in melting point and melt viscosity from the isocyanate compound, thorough and satisfactory kneading thereof may fail to be attained in a molding apparatus. Therefore, in the technique disclosed in the above publication, the effect of moisture on a cover material and the amount of an additive is not satisfactorily controlled, resulting in failure to produce a golf ball cover which is satisfactory in terms of improvement of scuff resistance.
Japanese Patent Application Laid-Open (kokai) No. 11-178949 discloses an aliphatic isocyanate-based thermoplastic polyurethane material to be used as a desirable thermoplastic polyurethane material. However, since the thermoplastic polyurethane material is highly reactive with isocyanate and its reaction is difficult to control, the polyurethane material involves the following problems: gelation easily occurs before injection molding, and sufficient plasticity cannot be maintained; gelation may occur during molding of a cover; and the polyurethane material cannot be recycled, due to gelation. Because of such problems, the thermoplastic polyurethane material is difficult to use in practice.
Japanese Patent Publication (kokoku) No. 58-2063 (U.S. Pat. No. 4,347,338) discloses a process for producing a thermosetting polyurethane product, in which a compound having two or more isocyanate groups is mixed with a thermoplastic resin which is non-reactive with an isocyanate group, the resultant mixture is incorporated into a thermoplastic polyurethane material, and the resultant material is subjected to molding by use of a molding machine. However, the purpose of the technique disclosed in the above publication is to improve the polyurethane product only in terms of solvent resistance and durability against continuous, repeating friction, and the publication does not disclose use of the aforementioned forming material as a material of a golf ball cover. There still exists demand for a golf ball cover material which can provide a golf ball with various necessary properties, such as restitution, total distance, spin performance, controllability, feeling on impact, scuff resistance, cut resistance, and discoloration resistance.